1. Field of the Invention
The present invention relates to high modulus, rigid chain polymers characterized by a capability of forming a cholesteric mesophase.
2. Description of the Prior Art
Para-linked aromatic polyamides have found commercial interest due to the fact that ultra-high-modulus fibers can be spun from their anistropic solutions. For example, poly-p-benzamide will exhibit a nematic phase when dissolved in H.sub.2 SO.sub.4 or N,N-dimethyl acetamide LiCl provided that the concentration equals or exceeds certain critical values.
For instance, oriented, ultra-high modulus fibers produced from nematic polymeric materials have been described in such prior art as, U.S. Pat. No. 3,671,524 which discloses the production of an isotropic nematic aromatic polyamide from which high modulus fibers could be spun. These fibers are characterized by a quiescent nematic phase which is local in character wherein the director vector, representing the direction of molecular alignment of the crystal phase, varies randomly from one domain to the next. While these fibers can be spun to obtain a uniaxial orientation in which essentially all the director vectors are aligned along the fiber axis, thereby imparting uniaxial orientation and single direction, high strength properties, these fibers are not capable of biaxial orientation.
There is an intrinsic difficulty if one tries to prepare a sheet or film from a nematic phase and still retain the exceptional modulus properties biaxially. Any attempt to orient the nematic phase produces uniaxial orientation, and even this orientation can only be achieved in films which are fairly thin and not too wide. Theoretically, we suppose, one could try to obtain biaxial high strength properties by lamination of multiple layers of nematic polymer material one on top of the other (See, J. C. Halpin, Polym Eng.Sci., 15, 132 (1975)), but in practice, it would seem that a good laminate would be quite difficult to obtain. Uniaxially oriented films are generally anisotropic with respect to such properties as strength and coefficient of thermal expansion. Thus, differences in thermal expansion between layers of a laminate would tend to preclude obtaining a durable adhesive bond. Thus, no successful laminate structure would be probable.
A cholesteric liquid crystal, or mesophase, is a twisted nematic in which the director vector, representing the average direction of long axis of the molecule in a given region, is rotated through a fixed angle upon passing from one nematic layer to the next. Low molecular weight compounds, as exemplified by certain esters of cholesterol, were known since before the turn of the century. Even cholesteric polymers are well known. No known cholesteric polymer, however, has the intrinsic molecular chain inflexibility and resistance to high temperatures to exhibit useful ultra high strength/high modulus properties. For instance poly(gamma-benzyl-L-glutamate) is a known cholesteric polymer, but it possesses a flexible helical chain configuration which is not suitable for high modulus applications. Polymers exhibiting a cholesteric mesophase have also been synthesized by placing a cholesteryl ester in the side chains. Several examples of such cholesteric polymers are described in V. P. Shibaev, et al, Dokl. Phys. Chem., 227, 400 (1976), E. C. Hsu, et al, J. Polym. Sci., Polym. Lett. Ed., 15, 545 (1977), and H. Finkelmann, et al, Makromol. Chem. 179, 829 (1978). In all those references, however, the main chains are flexible, and therefore do not exhibit outstanding mechanical properties. Aqueous hydroxypropyl cellulose solutions exhibiting a cholesteric phase are disclosed in I. S. Werbowyj et al Mol. Cryst. Liquid Cryst. Lett., 34, 97 (1976); however, the melting point of this polymer (150.degree. C.) is too low to make it of interest for high modulus applications.
It is also known that the addition of an optically active compound to a nematogenic compound, wherein both are of low molecular weight (i.e., non polymeric), produces a mixture exhibiting a cholesteric mesophase. For example, the addition of optically active d-tartaric acid to nematogenic p-n-octyloxybenzoic acid produces a mixture which exhibits a cholesteric phase (A. D. Buckingham et al. Chem. Phys. Letters, 3, 540 (1969). Moreover, the transformation of polymeric nematic phases into cholesteric phases by the addition of an optically active compound of low molecular weight has been disclosed by M. Panar et al, Macromolecules, 10, 1401 (1977). In the Panar et al publication, for example, a cholesteric phase is produced by the addition of optically active (+)-2-methylcyclohexanone to a nematic 20% solution of poly(1,4-benzamide). In that composition, the methylcyclohexanone is not covalently bonded to the polymeric phase but is only weakly associated therewith. The use of an unbonded low molecular weight additive seems to lead to a reduction of the desired high modulus properties, and the fugitive nature of the low molecular weight reduces the ability of such a system to form a desirable high modulus sheet for film.
The prior art is replete with disclosure of generically disclosed polymers which, if properly selected could form nematic mesophases. Such polymers have also been generically disclosed to contain a comonomer, which, if properly selected, could be optically active. However, none of those prior art references seems to have used isomers from a resolved racemic mixture, or at least never recognized specifically the use of monomers with active chiral centers individually in the D- or L- form. For instance, Morgan U.S. Pat. No. 3,991,016 discloses poly(1,4-benzamide) copolymers, some of which include chiral centers. However, there is no recognition of using specifically a combination of a nematic polymer, and an optically active monomer unit. Thus, none of the prior art references contain any recognition of the possibility of creating a cholesteric mesophase. Other prior art references with similar disclosures include Caldwell U.S. Pat. No. 3,408,334 and U.S. Pat. No. 2,956,977, and Schmitt et al U.S. Pat. No. 3,352,836.